[PATCH v3 17/18] selftests/mm: add memory-pressure threads to the khugepaged race harness

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>

The harness races collapse against faults, pins, fork, mremap and
MADV_DONTNEED, but nothing in it elevates a source folio's refcount from
the reclaim or compaction side.

Add two more threads, and run every mode and occupancy limit both with and
without them:

 - pageout: cycles MADV_PAGEOUT over a dedicated neighbour region, faults
   it back in and checks the content each round, since a page's pattern
   must survive the trip through swap.  Idle when the host has no swap,
   because then there is no anon reclaim to drive.

 - compactor: writes /proc/sys/vm/compact_memory in a loop.  Compaction
   isolates and migrates folios, so it competes with a collapse for the
   pages it is gathering, with refcount elevations and migration entries
   of its own.

-p narrows a run to the combinations that have them, the way -z narrows
the occupancy and -m the mode.  Each result says which it ran:

  ok 2 stepped/strict/pressure: 5s, 88 steps, no corruption

Assisted-by: Claude-Code:claude-opus-5
Tested-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
---
 tools/testing/selftests/mm/khugepaged_race.c | 158 +++++++++++++++++--
 1 file changed, 144 insertions(+), 14 deletions(-)

diff --git a/tools/testing/selftests/mm/khugepaged_race.c b/tools/testing/selftests/mm/khugepaged_race.c
index ba4f5c36bf88..bde36af03e49 100644
--- a/tools/testing/selftests/mm/khugepaged_race.c
+++ b/tools/testing/selftests/mm/khugepaged_race.c
@@ -18,6 +18,12 @@
  * zero-filled, and re-checked under the page table lock at install time in
  * case a racing fault got there first.
  *
+ * -p adds memory pressure to any of the above: MADV_PAGEOUT cycling
+ * on a dedicated neighbor region (swap traffic and LRU churn; skipped
+ * with a note when the host has no swap) and a compact_memory trigger
+ * loop (compaction migrates source folios, racing collapse's freeze
+ * with refcount elevation and migration entries of its own).
+ *
  * Correctness signals: every racing page must read as its pattern or
  * zero (MADV_DONTNEED), never anything else.  The faulters and the fork
  * children check that continuously, a final sweep checks it once more, plus
@@ -61,6 +67,8 @@ static unsigned long page_size;
 static char *region;		/* NR_AREAS * hpage_pmd_size */
 static char *mremap_area;	/* region + NR_SHARED_AREAS areas */
 static char *mremap_scratch;	/* well above the region */
+static char *pageout_area;	/* -p: dedicated pressure region */
+static size_t pageout_size;
 static int gup_fd = -1;
 static volatile int stop;
 static volatile int corrupted;
@@ -219,6 +227,70 @@ static void *mremapper_fn(void *arg)
 	return NULL;
 }
 
+/*
+ * -p: swap traffic and LRU churn on a region of our own. The content
+ * check is exact: a page out and back through swap must preserve the
+ * pattern, and nothing else ever writes here.
+ */
+static void *pageout_fn(void *arg)
+{
+	unsigned int seed = (unsigned long)arg;
+	unsigned long nr = pageout_size / page_size;
+	unsigned long i;
+
+	for (i = 0; i < nr; i++)
+		*(unsigned int *)(pageout_area + i * page_size) = pattern(i);
+
+	while (!stop) {
+		madvise(pageout_area, pageout_size, MADV_PAGEOUT);
+		for (i = 0; i < nr && !stop; i++) {
+			unsigned int val = *(unsigned int *)(pageout_area +
+							     i * page_size);
+
+			if (val != pattern(i)) {
+				corrupted = 1;
+				ksft_print_msg("Pageout corruption at page %lu: %#x != %#x\n",
+					       i, val, pattern(i));
+			}
+		}
+		usleep(rand_r(&seed) % 2000);
+	}
+	return NULL;
+}
+
+/* -p: compaction migrates the collapse sources out from under us. */
+static void *compactor_fn(void *arg)
+{
+	unsigned int seed = (unsigned long)arg;
+	int fd = open("/proc/sys/vm/compact_memory", O_WRONLY);
+
+	if (fd < 0) {
+		ksft_print_msg("No compact_memory; compactor idle\n");
+		return NULL;
+	}
+	while (!stop) {
+		if (write(fd, "1", 1) < 0)
+			break;
+		usleep(10000 + rand_r(&seed) % 100000);
+	}
+	close(fd);
+	return NULL;
+}
+
+static bool swap_available(void)
+{
+	char line[256];
+	int lines = 0;
+	FILE *fp = fopen("/proc/swaps", "r");
+
+	if (!fp)
+		return false;
+	while (fgets(line, sizeof(line), fp))
+		lines++;
+	fclose(fp);
+	return lines > 1;
+}
+
 static unsigned long now_ms(void)
 {
 	struct timeval tv;
@@ -230,11 +302,14 @@ static unsigned long now_ms(void)
 static void usage(void)
 {
 	fprintf(stderr,
-		"Usage: khugepaged_race [-d seconds] [-m stepped|free|madvise] [-z] [-a areas]\n"
+		"Usage: khugepaged_race [-d seconds] [-m stepped|free|madvise] [-z] [-p] [-a areas]\n"
 		"\tWithout -m, every mode runs in turn.\n"
 		"\t-d: seconds per mode (default 5)\n"
 		"\tBoth occupancy limits run unless -z asks for holes only.\n"
 		"\t-z: only max_ptes_none = HPAGE_PMD_NR - 1 (hole-heavy)\n"
+		"\tRuns with and without memory pressure unless -p asks for\n"
+		"\tpressure only.\n"
+		"\t-p: only with the pageout and compaction threads\n"
 		"\t-a: number of shared PMD-sized playground areas (default 3)\n");
 	exit(1);
 }
@@ -243,18 +318,22 @@ int main(int argc, char **argv)
 {
 	static const char * const thread_names[] = {
 		"faulter", "faulter2", "dontneed", "pinner", "forker",
-		"mremapper",
+		"mremapper", "pageout", "compactor",
 	};
 	void *(*const thread_fns[])(void *) = {
 		faulter_fn, faulter_fn, dontneed_fn, pinner_fn, forker_fn,
-		mremapper_fn,
+		mremapper_fn, pageout_fn, compactor_fn,
 	};
+	const unsigned long pageout_bit = 1UL << 6, compactor_bit = 1UL << 7;
 	const int nr_threads = ARRAY_SIZE(thread_names);
 	pthread_t threads[ARRAY_SIZE(thread_names)];
 	static const char * const all_modes[] = { "stepped", "free", "madvise" };
 	static const int all_nones[] = { 0, 1 };	/* strict, holes */
+	static const int all_press[] = { 0, 1 };	/* quiet, under pressure */
 	const int *nones = all_nones;
+	const int *press = all_press;
 	int nr_nones = ARRAY_SIZE(all_nones);
+	int nr_press = ARRAY_SIZE(all_press);
 	const char *one_mode[1];
 	const char * const *modes = all_modes;
 	int nr_modes = ARRAY_SIZE(all_modes);
@@ -263,13 +342,15 @@ int main(int argc, char **argv)
 	unsigned long end_ms;
 	int duration_s = 5;
 	unsigned long thread_mask = ~0UL;
+	unsigned long base_mask;
 	int nr_areas_arg = 0;
 	bool holes_only = false;
+	bool pressure_only = false;
 	unsigned long i;
 	int steps = 0;
 	int opt;
 
-	while ((opt = getopt(argc, argv, "a:d:m:t:zh")) != -1) {
+	while ((opt = getopt(argc, argv, "a:d:m:t:zph")) != -1) {
 		switch (opt) {
 		case 'a':
 			nr_areas_arg = atoi(optarg);
@@ -287,6 +368,9 @@ int main(int argc, char **argv)
 		case 'z':
 			holes_only = true;
 			break;
+		case 'p':
+			pressure_only = true;
+			break;
 		default:
 			usage();
 		}
@@ -296,6 +380,11 @@ int main(int argc, char **argv)
 		nr_nones = 1;
 	}
 
+	if (pressure_only) {
+		press = all_press + 1;
+		nr_press = 1;
+	}
+
 	if (mode_arg) {
 		if (strcmp(mode_arg, "stepped") && strcmp(mode_arg, "free") &&
 		    strcmp(mode_arg, "madvise"))
@@ -335,7 +424,12 @@ int main(int argc, char **argv)
 		 -1, 0) != (void *)mremap_scratch)
 		ksft_exit_fail_perror("mmap() mremap scratch");
 
-	ksft_set_plan(nr_modes * nr_nones);
+	base_mask = thread_mask;
+	if (!swap_available())
+		/* No swap, no anon reclaim: compaction-only pressure. */
+		ksft_print_msg("no swap: the pageout thread stays idle\n");
+
+	ksft_set_plan(nr_modes * nr_nones * nr_press);
 
 	thp_save_settings();
 	thp_read_settings(&settings);
@@ -347,9 +441,17 @@ int main(int argc, char **argv)
 	 */
 	thp_push_settings(&settings);
 
-	for (int mn = 0; mn < nr_modes * nr_nones; mn++) {
-		const char *mode = modes[mn / nr_nones];
-		bool holes = nones[mn % nr_nones];
+	for (int run = 0; run < nr_modes * nr_nones * nr_press; run++) {
+		int rem = run % (nr_nones * nr_press);
+		const char *mode = modes[run / (nr_nones * nr_press)];
+		bool holes = nones[rem / nr_press];
+		bool pressure = press[rem % nr_press];
+
+		thread_mask = base_mask;
+		if (!pressure)
+			thread_mask &= ~(pageout_bit | compactor_bit);
+		else if (!swap_available())
+			thread_mask &= ~pageout_bit;
 
 		thp_read_settings(&settings);
 		settings.thp_enabled = THP_MADVISE;
@@ -385,6 +487,24 @@ int main(int argc, char **argv)
 			ksft_exit_fail_perror("mmap() playground");
 		mremap_area = region + nr_shared_areas * hpage_pmd_size;
 
+		if (thread_mask & pageout_bit) {
+			/*
+			 * Big enough to cycle real reclaim, small enough not
+			 * to dominate a TCG guest: 4 PMD areas, clamped to
+			 * [16M, 64M].
+			 */
+			pageout_size = 4 * hpage_pmd_size;
+			pageout_size = pageout_size < (16UL << 20) ?
+				       (16UL << 20) :
+				       pageout_size > (64UL << 20) ?
+				       (64UL << 20) : pageout_size;
+			pageout_area = mmap(NULL, pageout_size,
+					    PROT_READ | PROT_WRITE,
+					    MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+			if (pageout_area == MAP_FAILED)
+				ksft_exit_fail_perror("mmap() pageout area");
+		}
+
 		/* Populate so the first pass has something to collapse. */
 		for (i = 0; i < nr_shared_areas * hpage_pmd_size / page_size; i++)
 			*(unsigned int *)(region + i * page_size) = pattern(i);
@@ -434,8 +554,9 @@ int main(int argc, char **argv)
 			check_page(i);
 
 		ksft_test_result(!corrupted,
-				 "%s/%s: %ds, %d steps, no corruption\n",
+				 "%s/%s%s: %ds, %d steps, no corruption\n",
 				 mode, holes ? "holes" : "strict",
+				 pressure ? "/pressure" : "",
 				 duration_s, steps);
 
 		/*
@@ -444,17 +565,26 @@ int main(int argc, char **argv)
 		 * and its scan cadence differs.
 		 */
 		munmap(region, nr_areas * hpage_pmd_size);
+		if (pageout_area) {
+			munmap(pageout_area, pageout_size);
+			pageout_area = NULL;
+		}
 		thp_pop_settings();
 		stop = 0;
 		steps = 0;
 
 		if (corrupted) {
 			/* Memory is suspect; the rest would prove nothing. */
-			while (++mn < nr_modes * nr_nones)
-				ksft_test_result_skip("%s/%s: skipped after corruption\n",
-						      modes[mn / nr_nones],
-						      nones[mn % nr_nones] ?
-						      "holes" : "strict");
+			while (++run < nr_modes * nr_nones * nr_press) {
+				rem = run % (nr_nones * nr_press);
+
+				ksft_test_result_skip("%s/%s%s: skipped after corruption\n",
+						      modes[run / (nr_nones * nr_press)],
+						      nones[rem / nr_press] ?
+						      "holes" : "strict",
+						      press[rem % nr_press] ?
+						      "/pressure" : "");
+			}
 			break;
 		}
 	}
-- 
2.54.0